U.S. Pat. Nos. 4,918,128 and 5,190,818, provide pressure sensitive adhesives/cohesives that are revolutionary in their utility when utilized with mailers or like type documents or business forms. These patented adhesives have allowed the practical implementation in the mailer industry of mailers which. may be quickly and effectively produced and sealed without the drawbacks associated with conventional heat or moisture activated adhesives.
While the adhesives in the above identified patents are extremely effectively, it is desirable to be able to produce an adhesive that has even better properties. While the adhesives in the above identified patents are not tacky after the application to paper, and do not adhere/seal unless a high level of pressure is applied (which prevents the adhesive from blocking during the printing, which would slow down the processing due to paper jams or the like), sometimes the adhesive action may be hampered by contamination of the printing surface and adhesive with silicone lubricants used in conventional printers (particularly laser printers). The adhesive according to the present invention, when coated on paper or like substrates used in mailer type business forms, or the like, has the same advantageous properties as the adhesives in the above identified patents and also has enhanced functionality when contaminated by silicone lubricants.
Like the preferred embodiments in the above identified patents, the cohesive according to the present invention contains a natural rubber graft, an appropriate acrylate monomer or monomers such as methyl methacrylate (or cyclohexyl methacrylate, or benzyl methacrylate, or isobornyl methacrylate), finely divided hard particles such as silica, and preferably also starch. The cohesive according to the present invention also includes acrylic acid and/or 4-acetoxystyrene, and optionally ethyl hexyl acrylate.
According to the present invention a pressure sensitive adhesive/cohesive is provided comprising the following components: 100 parts by weight natural rubber. About 5-35%, by weight of the rubber, acrylate monomer. About 0.5-8%, by weight of the rubber, acrylic acid and/or about 1-10% by weight of rubber 4-acetoxystyrene. About 0-20%, by weight of the rubber, ethyl hexyl acrylate. 1-50%, by weight of the rubber, a finely divided hard particulate material having substantially no thermoplasticity. 0-50%, by weight of the rubber, starch. And 0-40%, by weight of the rubber, carboxylated styrene-butadiene latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex or pre-crosslinked natural rubber latex, or a tackifier, or combinations thereof.
In the case of 4-acetoxystyrene as a comonomer in the graft/block of the rubber, grafted rubber has built-in antioxidant as a result of hydrolysis of acetoxy group which provides the cohesive antioxidant property helping its shelf life as a coated cohesive by acting as a free radical scavenger.
The pressure sensitive adhesive/cohesive according to the present invention is typically coated on. a piece of paper. More typically, since the material according to the invention is best as a cohesive, it is coated on two portions of a piece of paper, the cohesive portions in contact with each other as a result of folding the paper (such as by forming a mailer), or bringing a like sheet in contact therewith, and pressure sealed together (such as by running through conventional pressure sealing equipment, such as xe2x80x9cSpeedisealer(copyright)xe2x80x9d equipment available from Moore USA of Lake Forest, Ill.), so as to cause paper fiber tear if attempted to be pulled apart.
Preferably, the adhesive/cohesive comprises at least 1% ethyl hexyl acrylate, and at least 1% starch; e.g. about 10-30% starch (such as about 20% starch) having an average particle size of about 5-25 microns. The hard particulate material preferably comprises silica gel, e.g. about 10-30% (such as about 20%), the silica gel having an average particle size of about 0.2-20 microns (e.g. about 0.3-0.4 microns), or a fumed silica (e.g. 0.1xcx9c0.3 microns). There also may be at least 5% carboxylated styrene-butadiene latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex, or pre-crosslinked natural rubber latex, or a styrene-acrylateacrylonitrile latex, or a tackifier, or combinations thereof. The acrylic monomer preferably comprises methyl methacrylate, e.g. about 5-30%, or 0.5-8% of 4-acetoxystyrene, cyclohexyl methacrylate, benzyl methacrylate, or isobornyl methacrylate; and the acrylic acid typically is between about 0.5-4%, and the 4-acetoxystyrene between about 1-3%. The modified natural rubber latex is preferably electrosterically stabilized natural rubber graft and block terpolymer.
According to another aspect of the present invention a method of making a mailer type business form is provided. The method comprises: (a) applying cohesive as cooperating patterns cohesive to a sheet of paper so that when the sheet is folded, or brought into contact with a like sheet, the patterns move into contact with each other, the cohesive comprising a pressure sensitive cohesive including: 100 parts by weight natural rubber; about 5-35% by weight of the rubber acrylate monomer; about 0-8% (preferably about 0.5-8%) by weight of the rubber acrylic acid and/or about 0-10% (preferably about 1-10%) by weight 4-acetoxystyrene; about 0-20% by weight of the rubber ethyl hexyl acrylate; 1-50% by weight of the rubber a finely divided hard particulate material having substantially no thermoplasticity; 0-50% by weight of the rubber starch; and 0-40% by weight of the rubber carboxylated styrene-butadiene latex, styrene-acrylate-acrylonitrile latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex, or pre-crosslinked natural rubber latex, or a tackifier, or combinations thereof.
The method may comprise the further steps of: (b) folding the paper to move the cohesive patterns into contact with each other, and (c) applying a sealing pressure of at least about 100 lbs/lineal inch (e.g. about 200 lbs/lineal inch) to the patterns to seal the cohesive together so as to cause fiber tear if attempted to be pulled apart.
It is a primary object of the present invention to provide a pressure sensitive cohesive/adhesive which exhibits excellent adhesive bonding, cohesive bonding, resistance to heat, blocking resistance, resistance to abrasion, non-tackiness, good creep properties, and substantially no loss in adhesion/cohesion upon exposure to heat and polysiloxane based lubricants. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.